Influence of supercritical CO2 on fracture network evolution with nanopore structure linkage in high-rank anthracite matrix

It is essential to understand the permeability evolution of coal reservoirs under the interaction of supercritical CO2 (SC-CO2) in order to be successful in both CO2 geological storage and extraction of coalbed methane. However, the influence of nanopore structure characteristics on the permeability evolution of coal exposed to SC-CO2 has not been elucidated. To this end, the quantitative analysis of the pores in high-rank coal was conducted through low-temperature N-2 adsorption, low-temperature CO2 adsorption, and Nuclear Magnetic Resonance (NMR). The analysis revealed that micropores and mesopores are the major contributors to the specific surface area and pore volume of the nanopores, respectively. After SC-CO2 interaction, the pore size and pore number of the coal increased significantly, with small pores changing to large ones, mainly due to the dissolution of SC-CO2 with the contribution of gas pressure being negligible. The proportion of micropore and mesopore decreases while that of macropore increases after SC-CO2 treatment, especially SC-CO2 causes macropores to be continuous and increased the connectivity of fractures. The relationship between nanopore structure characteristics and the deformation of the fracture network under SC-CO2 interaction was studied. The results showed that the specific surface area (SSA) of the micropore and the volume fractal dimension (D-LN2) of the mesopore were related to the change of fracture porosity, while the surface fractal dimension (D-LN1) of the micropore, the specific surface area (SSA) and pore volume of mesopore were related to the change of fracture complexity. This indicated that the evolution of the fracture network under SC-CO2 interaction is related to the nanopore structure characteristics. A mechanical model was established based on the close correlation between nanopore structures characteristics in the matrix and the evolution of fracture networks under SC-CO2 interaction. This mechanical model elucidates the impact of nanopore alteration resulting from mineral dissolution induced by SC-CO2 on the evolution of fracture networks. This research provides a clearer understanding of the evolution in reservoir permeability during CO2 geological sequestration, offering valuable reference for safety assessments.